There are two types of titanium dioxide pigments; both have a tetragonal crystal form. One crystal form is anatase (a soft crystal), and the other is rutile (a hard crystal).
Anatase, which comprises nearly half of the market, is used in the paper, textile fiber and plastic industries where a soft pigment will not wear away the cutting knives, extruders and injection equipment. The sulfate process is amenable to the manufacture of anatase pigments. The vapor phase chloride process is limited to the production of rutile. The prior process employing sulfates produced a large amount of pollution. Development of this new process eliminates most of the pollution.
For a better understanding of the invention, this sulfate process is described.
Drying and Grinding
Ilmenite ore or titanium slag (containing 1 to 5% humidity as received in a plant) is first dried to 0.3% humidity. It is then ground to from an average of 80 to 98% minus 200 mesh.
Digestion
A typical Ilmenite digestion reacted at 85% reaction acid is described as follows. 71,000 pounds of 66.degree. Baume sulfuric acid are charged into a reactor, and air agitation is started. 45,000 pounds of Ilmenite ground ore are slowly added (over 30 minutes), and then 6700 pounds of acceleration water are added. A reaction starts at from 95.degree. to 105.degree. C., and the temperature in the reactor rises rapidly from 120.degree. to 200.degree. C. with the evolution of 10,000 to 12,000 pounds of steam (with entrained acids), which is vented to the atmosphere, causing so-called acid rain, the reaction is often violent, emptying to the atmosphere with explosive force. When the reaction is complete, the resulting porous mass is allowed to cool, and water is added to dissolve soluble iron and titanium sulfates. Metallic iron is added to reduce any ferric sulfate formed, and the thus-obtained slurry (suspension) is pumped to settling tanks. This operation takes from about 12 to 24 hours.
Settling and Washing
Due to the presence of rutile in the ground ore, gangue minerals and colloids (mostly silica gel formed when digesting ore or slag) the suspension cannot be filtered directly, and it is pumped to settling tanks where the clear supernatant liquor and settled solids (mud) are obtained. The settled solids are again dispersed and washed counter-currently in a series of settling tanks to recover a maximum amount of titanium sulfate. The number of required tanks and their size take considerable land and entail significant cost.
Filtration
The combined clear settled liquors from the first settlers (together with leach liquor) are filtered and led to a vacuum evaporator (when using ilmenite ore) or directly to a hydrolysis process (when using slag, which contains less iron).
Evaporation under vacuum at temperatures below 60.degree. C. is necessary to prevent premature hydrolysis. Copperas from a vacuum evaporator crystallizer is filtered, washed and dumped to sewage or sold.
A typical analysis of the concentrated filtered solution ready for hydrolysis is:
______________________________________ Analyis Typical Value ______________________________________ Sp. Gr. 1.620 at 60.degree. C. % TiO.sub.2 15.4 % FeSO.sub.4 10.8 % H.sub.2 SO.sub.4 27.7 ______________________________________
Hydrolysis
The solution is then heated to a temperature of 96.degree. C., and hydrolysis is started by pumping part of the liquor into 96.degree. C. hot water (about 1/4 of the liquor volume) over a period of 16 minutes. The mixture is then brought to a boil (102.degree. C. to 108.degree. C.). Boiling is continued until about 96% of the titanium is recovered.
The resulting suspension is led to a series of counter-current filter arrangements (e.g., Moore filters), in which the titanium hydrate is separated and washed. The resulting solution (containing weak sulfuric acid) is discharged to sewage.
The preceding process causes significant pollution, which is the reason why it is being abolished in all industrialized countries.